The I-CSCF will then forward the request
to the S-CSCF assigned to the subscriber. This allows the P-CSCF to maintain the state
of the subscriber and that subscriber??™s registration.
Anytime there is a change in the registration status of a subscriber, the NOTIFY
method is used to alert all subscribed entities of the change. For example, the network
may de-register a subscriber from the network. This will result in a NOTIFY being sent
to all subscribed entities, providing a list of all public user identities for the subscription
being de-registered.
Thus far we have discussed accessing the IMS network and registration procedures.
Accessing the IMS allows devices to quickly establish their location and notify the
network of their location. This is assuming that the subscriber device is already SIP
capable and compatible with IMS. However, this is not yet the case, and therefore subscribers
will have to access the IMS through legacy networks. This means there are additional
entities required to convert the call control data to SIP for IMS compatibility.
This next section describes the procedures through other non-IMS networks for gaining
access and establishing sessions. We will then explore how a session is established
and managed using SIP.
Interworking with the PSTN
As mentioned, operators will depend on their existing legacy networks to provide services
to their subscribers.
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